The invention relates to magnetic nerve stimulators and, more specifically, to a stimulator capable of producing a rapid train of large amplitude, short duration magnetic pulses.
Nerve stimulation and stimulators are important diagnostic tools in medicine today. Both electrical and magnetic means are employed to stimulate brain tissue and peripheral nerves. Existing stimulators are useful for exploring cognitive phenomenon, measuring central nerve conduction velocity, and intraoperative monitoring of spinal cord trauma during surgery. However, existing stimulators are inadequate for use in detailed mapping of the brain's functional (e.g., speech, vision, etc.) areas, peripheral nerve stimulation in situations requiring comfort and relaxation, and intraoperative monitoring of the front of the spinal cord.
Patients with seizures or brain tumors may require that a portion of their brain be removed. For example, if medical treatment of epilepsy patients is ineffective, an alternative is to remove that portion of the brain containing the epilepsy seizure focus. However, the seizure focus may be located next to a functionally important area of the brain.
To tailor the operation for removal of the seizure focus only, the brain is mapped in specialized epilepsy units using electrical stimulation. Generally, two mappings of the brain are made, one to locate the seizure focus, the second to locate nearby functional areas. For example, to locate the brain's functional area responsible for speech, two techniques are currently used prior to seizure surgery. The first consists of removing portions of the scalp and skull to permit placement of electrodes in a grid pattern directly on the brain. Electrical stimulation is then used to generate a mapping. The second technique consists of putting one half of the brain to sleep with an injection in order to determine which half controls speech. While both techniques are useful, they are not always successful and, most importantly and obviously, they are very invasive.
Certain patients with brain tumors also require brain mapping to preserve functional areas from being damaged during surgery to remove the tumor. While magnetic resonance imaging can locate the tumor, it is an anatomical test and does not provide information on the function of areas of the brain located close to the tumor.
Peripheral nerve stimulation is used to define nerve injuries and/or malfunction resulting from trauma, back injury, diabetes, peripheral neuropathies, etc. This is currently done by electrical devices which by giving repetitive shocks to nerve can be used to measure velocity and amplitude of signals carried by the nerve. For example, if a patient's hand is numb due to nerve damage from an industrial accident, an electrical stimulator can be used to determine nerve function near the elbow and, hence, the extent and location of the injury to the nerves.
A problem with using electrical stimulation in testing peripheral nerves is that the electrical shocks while tolerable are uncomfortable. Also, a pulse rate which is tolerable is insufficient to obtain good results for a subset of tests that are used in peripheral nerve stimulation.
Nerve stimulation is used in intraoperative monitoring to warn a surgeon if he or she is adversely affecting the spinal cord or brainstem. An electrical stimulator is used to obtain the somatosensory evoked potential by stimulating the peripheral nervous system. Stimuli to skin or nerve will evoke nerve signals that travel to the cortex region of the brain and the response to the stimulus is recorded, in either a continuous or intermittent fashion, during this process. If the surgeon compresses the spinal cord too much, e.g., during decompression of a disk, the response signals will disappear giving the surgeon an opportunity to readjust. The problem with somatosensory evoked potentials, however, is that they only test the back part of the spinal cord and it is the front part that is most likely to be damaged during many types of surgery.
A method for testing the front part of the spinal cord is to use a magnetic stimulator near the motor control region of the brain. This will obtain a motor evoked potential (MEP) which is the reverse of an evoked potential in that an MEP signal travels from the brain to the periphery rather than from the periphery to the brain. Most importantly, the MEP signal travels from the brain down the front of the spinal cord. Unfortunately, existing magnetic stimulators pulse at most once per second which is too slow to obtain sufficient numbers of MEPs to provide immediate feedback to the surgeon.
Current magnetic nerve stimulators (FIG. 1) operate by discharging a large electric charge stored in a capacitor through a coil placed near the head or nerves of a human or animal. They are an improvement over electrical stimulators because they can be used in brain mapping without opening the skull, i.e., noninvasively; they are significantly less painful; and, as noted above, they can be used to generate motor evoked potentials to test the front part of the spinal cord as part of intraoperative monitoring.
However, shortcomings remain. As noted with intraoperative monitoring, the pulse repetition rate is extremely low. This results from the need to recharge the energy storing capacitor. Recharging is usually done through a high voltage DC power supply with a resistor or resistor-inductor circuit to limit the peak current demand from the power supply.
Slow pulse repetition rate precludes the possibility of disruption of high level cognitive function required for proper brain mapping. Present stimulators are only useful for producing motor twitching, motor evoked potentials, or brief visual stimulation. High order functions such as speech, memory, or behavior cannot be disrupted by slow pulse rates. This makes it impossible to perform cognitive functional mapping such as language mapping with these devices. The slow pulse repetition rate also results in very long times being needed to perform the signal averaging necessary to measure nerve potentials.
A final problem with present magnetic stimulators arises from the large amounts of heat which are dissipated in the stimulating coil which is placed near the body. Nearly all of the energy of each pulse is converted to heat in the coil during the stimulation pulse. Coil heating limits the time of operation as well as the pulse repetition rate in order to avoid burning the subject or damaging the apparatus.
In sum then, what is needed is a magnetic stimulator that can be used to map functional areas of the brain effectively and noninvasively; that can stimulate peripheral nerves less painfully and more rapidly; and that can pulse rapidly enough to obtain sufficient motor evoked potentials to be an effective intraoperative monitor.